The present invention generally relates to a method of casting a metal article having a long thin portion. More particularly the present invention relates to a method of casting thinwall metal articles that is single-cast, allows for high temperatures, is quick, relatively inexpensive, not labor intensive, improves current quality and may use traditional equiaxed investment casting technology.
It is known within the art to form thinwalls by providing metal which has been cold rolled to a very thin thickness. The cold-rolled metal is then etched or machined to provide small holes in the surface. The metal is then formed and bonded in a curved shape to produce the particular desired part. The forming process may result in the distortion of holes in the wall.
Long thin articles have also previously been cast with a directionally solidified or columnar grained crystallographic structure. Using this method, the mold is preheated to a relatively high temperature which is above the liquidus temperature of the metal of which the cast article is to be formed. Super heated molten metal is then poured into the preheated mold, with heat being supplied during pouring so that the metal remains molten during and immediately after pouring. The molds have gates placed at various locations along the length of the mold. This gating is used to conduct molten metal which compensates for the decrease in the volume of the metal during solidification.
The use of a directionally solidified or columnar grained crystallographic structure is known within the art. U.S. Pat. No. 4,724,891, incorporated herein by reference, provides a method of casting a thinwall using a directionally solidified furnace. A mold having a cavity with a configuration corresponding to the desired configuration of the cast is formed. The mold cavity has a thinwall forming portion in which major side surfaces of the mold are spaced apart by a distance, 0.05 inches or less, corresponding to the desired thickness of the thinwall portion of the article. After the mold has been preheated to a temperature close to the melting temperature of the metal which is to be cast, molten metal is poured into the mold. While the molten metal is being poured, the mold is heated so that molten metal can completely fill the mold cavity. After filling the mold cavity, the molten metal in the thinwall forming portion of the mold cavity is solidified to form a continuous solid body having a configuration corresponding to the configuration of the thinwall portion of the article. To prevent the formation of voids as the molten metal solidified, the molten metal is solidified in one direction by moving an interface between molted and solid metal in one direction through the thinwall forming portion of the mold cavity. The direction of solidification of the molten metal through the thinwall forming portion of the mold cavity is toward the gating or end portion of the mold may cavity into which the molten metal was originally conducted.
U.S. Pat. No. 6,244,327, herein incorporated by reference, discloses a method of making single-cast, high temperature thinwall structures having a high thermal conductivity member connecting the walls. This process controls the uses a pressurized, directionally solidified furnace and controls the injection pressure of an alloy in order to solve the problem of creep of a ceramic shell. This process is complicated in that it utilizes conductivity rods and/or connectivity pins.
Gas turbine engines are continually being improved, requiring higher operating temperatures. As an example, the combustion chamber, which to date has been required to maintain strength at temperatures up to about 1300-1400xc2x0 F., is now being designed to operate at temperatures of about 1600xc2x0 F. or greater where creep is critical. In the past, metal articles where manufactures from convention cobalt-based or nickel-based alloys. These materials were chosen because of their sufficient strength and physical characteristics. Many conventional designs are fabricated from sheet stock or alternatively machined from wrought material. However, these techniques can be relatively expensive due to the extensive working required to form the thinwalled shape and because of the number of structural welds. Also, directional solidification processes take a great deal of time, resulting in higher costs, and the metal gets too cold too quickly, resulting in large holes.
As can be seen there is a need for a method of producing thinwall structures that is single-cast, allows for high temperature, thinwall structures which is quick, relatively inexpensive, not labor intensive, improves current quality and may use traditional equiaxed investment casting technology.
In one aspect of the invention, a method is disclosed for casting a metal article comprising the steps of forming a mold having a mold cavity with a height, a thinwall portion and a base wall portion. The base wall portion may have a thickness less than 10 times the thickness of the thinwall portion and at least one gate. The thinwall portion may be less than 0.05 inches thick and free of gating. The method comprising the steps of: positioning the mold in a preheated furnace so that the furnace substantially surrounds the mold and so that a longitudinal axis of the long thin portion of the mold cavity is in an upright orientation; heating the mold in the furnace to a temperature between 1045xc2x0 C. and 1055xc2x0 C., and drawing a vacuum in the furnace. The metal is heated until it forms a molten metal and poured at a temperature between 1560xc2x0 and 1570xc2x0 C. into the mold cavity. The vacuum is broken and the mold cavity withdrawn from the furnace. The molten metal is allowed to solidify in the mold cavity solidify with a equiaxed grain structure.
In another aspect of the present invention a method for casting a metal articles is disclosed, comprising the steps of injecting investment casting wax into a metal wax injection die to form a wax assembly, repeatedly dipping the wax assembly in a ceramic slurry and allowing to dry at least two times. This results in a mold with a mold cavity having an outer diameter, height, thinwall portion, a base wall portion with at least two gates. The thinwall portion may be less than 0.05 inches thick and free of gating. The mold is then cleaned, burned and coated. Then the mold may be positioned in a preheated furnace so that the furnace substantially surrounds the mold and so that a longitudinal axis of the long thin portion of the mold cavity is in an upright orientation. The mold is heated to a temperature that may be between 1045 and 1055xc2x0 C. A vacuum is drawn in the furnace and a metal heated until it forms a molten metal. The metal may then be poured into the mold cavity. The vacuum is then broken and the mold removed. Inside the mold the molten metal is allowed to solidify so as to form a metal article with a equiaxed grain structure.
In yet another aspect of the present invention a method is disclosed for casting a metal combustion case, comprising the steps of injecting investment casting wax into a metal wax injection die to form a wax assembly, washing said wax assembly, drying the wax assembly, dipping the wax assembly in a primary coat, allowing the primary coat to dry, dipping the wax assembly in at least one backup coat, allowing the backup coat to dry, resulting in a mold with a mold cavity having an outer diameter between 12 and 22 inches, a height between 5 and 10 inches, a thinwall portion with a thickness less than 0.35 inches and free of gating, a base wall portion with a thickness less than 0.30 inches with twelve gates and twelve atomisers, wherein the gates may be between the twelve atomisers. The mold may then be placed in a Boilverclave, then in a Dewax kiln. The mold may then coated with a final coat and wrapped in Kerlane. The mold may be positioned in a preheated furnace so that the furnace substantially surrounds the mold and so that a longitudinal axis of the long thin portion of the mold cavity is in an upright orientation and heated to a temperature between 1045xc2x0 C. and 1055xc2x0 C. A vacuum may be drawn, and a nickel based alloy metal heated until it forms a molten metal, this molten metal may be poured at a temperature between 1560xc2x0 C. and 1570xc2x0 C. into the mold cavity, the vacuum broken and the mold withdrawn. The molten metal may be allowed to solidify so as form a combustion case metal article with a equiaxed grain structure.
In yet another aspect of the present invention, a method for casting a metal combustion case for use within a gas turbine engine is disclosed comprising the steps of injecting investment casting wax into a metal wax injection die to form a wax assembly, washing the wax assembly, drying the wax assembly, dipping the wax assembly in a primary coat, allowing the primary coat to dry, dipping the wax assembly in at least one backup coat and allowing said backup coat to dry. This results in a mold with a mold cavity having a height, a thinwall portion less than 0.05 inches thick and free of gating and a base wall portion with a thickness less than 10 times the thickness of the thinwall portion and having at least one gate. The mold is placed in a Boilverclave, then in a Dewax kiln. Then the mold may be coated with a final coat and wrapped in Kerlane. The mold may then be positioned in a preheated furnace so that the furnace substantially surrounds the mold and so that a longitudinal axis of the long thin portion of the mold cavity is in an upright orientation. The mold is heated in said furnace to a temperature between 1045xc2x0 C. and 1055xc2x0 C., a vacuum may be drawn, and molten metal poured at a temperature between 1560xc2x0 C. and 1570xc2x0 C. into the mold cavity. The vacuum is then broken and the mold withdrawn, so as to cause said molten metal to solidify to form a metal article with a equiaxed grain structure.